Display units are known which create an image in space. Such display units typically comprise a light source, a beam splitter comprising a sheet of titanium coated glass, a layer of retroreflective material and a stencil so arranged that, in use, light passing through the stencil causes an image of the stencil to be formed in space. WO 83/03019 discloses one such display unit.
Whilst such display units provide an interesting visual effect they do not catch the eye, nor do they hold an observer's attention for more than a few seconds.